The laser beam provides a means of writing, bar coding and decorative marking of plastics. This technique is advantageous over current printing technologies because of the ease at which the layout can be adjusted using graphic computer programs and also integrated into the production line. Laser marking enables a contact-free procedure even on soft, irregular surfaces that are not readily accessible. In addition it is ink-free which makes it long-lasting and solvent-free and, thus, more friendly to the environment. Speeds up to 10,000mm/sec are possible with a CO.sub.2 laser while Nd-YAG laser allows up to 2000 mm/sec.
There are several laser types available for marking plastic surfaces. The Excimer laser with the frequency in the range of 196-351 nm leads to the marking of plastic surfaces by photochemical ablation or reaction. Using Nd-YAG laser at lower power levels at 532 nm provides laser marking by leaching or selective bleaching of dyes and pigments while the NdYAG laser at 1064 nm leads to laser marking by carbonization, sublimation, discoloration, foaming and engraving. The CO.sub.2 laser at 10600 nm enables laser marking by thermochemical reaction, melting, vaporizing and engraving.
In one type of laser marking it is desirable to obtain a light contrast on a dark background. Carbon black which decomposes into volatile components after absorbing the laser light may be utilized. These volatile components foam the surface leading to scattering of light and thus a light impression. EP 0 675 001 to Kato describes the use of zinc borate as a contrast enhancing additive. Zinc borate releases its water. U.S. Pat. No. 4,595,647 to Spanjer describes a laser markable material useful for encapsulation of electronic devices that is obtained by adding TiO.sub.2 or TiO.sub.2 +CrO.sub.3 to common plastic encapsulants formed from a mixture of a resin+filler+carbon black+mold release agent. When irradiated by a CO.sub.2 laser, the originally grey material turns bright gold, providing a high contrast durable mark. Desirable concentrations are described, in weight percent of the compound, as 1-5% TiO.sub.2 and 0-3% CrO.sub.3, with 1-3% TiO.sub.2 and 0.5-2% CrO.sub.3 being preferred. Carbon black is optional but a concentration in the range 0.1-3% by weight is desirable with 0.5-1% preferred.
U.S. Pat. No. 5,063,137 to Kiyonari et al. describes the use of an anhydrous metal borate salt, an anhydrous metal phosphate salt, a phosphoric acid-containing glass, basic zinc carbonate and basic magnesium carbonate with a resin to give a white marking on a dark background.
It is also desirable to form a dark contrast on a light background. EP 0 111 357 uses metal silicates to obtain black markings on articles having a polyolefin surface. U.S. Pat. No. 4,578,329 to Holsappel describes the use of a silicon compound, preferably a metal silicate, e.g. calcium-metasilicate or kaoline to give a black mark in the laser struck areas of a polyolefin.
U.S. Pat. No. 5,489,639 to Faber et al describes the use of copper phosphate, copper sulfate and copper thiocyanate with a thermoplastic resin to give dark markings.
U.S. Pat. No. 4,816,374 to Lacomta describes the use of lead iodide, lead carbonate, lead sulfide, dioxin isocyanate, antimony; related compounds and mixtures with polyolefins.
T. Kilp, "Laser marking of Plastics', Annu. Tech. Conf. Soc. Plast. Eng, (1991), 49th, 1901-1903, describes the effects of different silicates on the laser marking of polyolefins. Kaolin gave white marks on colored substrates while black marks were obtained when mica or titanium dioxide were incorporated into the substrate.
It is desirable to make further improvements in laser marking materials of the polyester type. In particular, a desired color combination is a light background color and a dark contrast color in the laser treated areas. In particular, it is desirable to obtain a dark contrast color in the laser treated areas using a NdYAG laser.